There are some applications in which large diameter underwater pipes are required to transport large volumes of fluid under low pressures. One such application is in ocean thermal energy conversion (OTEC) plants.
An OTEC system uses the naturally occurring temperature difference between water at the surface of an ocean and water from the depths of the ocean to drive a power-producing cycle. As long as the temperature between the warm surface water and the cold deep water differs by about 20° C., an OTEC system can produce a significant amount of power.
A conventional OTEC system incorporates an electrical generation system, which is located at the surface of the water and produces electrical energy using the temperature differential between fluids in two heat exchangers. A first heat exchanger uses the heat from warm surface water to vaporize a fluid contained in a closed-loop conduit. The energy of the vaporized fluid is used to spin a turbine, which turns an electrical generator that generates electrical energy. After the vaporized fluid passes through the turbine, it is channeled by the conduit to the second heat exchanger. The second heat exchanger uses cold water piped up from the depths of the ocean to condense the vapor back to the liquid state.
The cold water received by the second heat exchanger is typically pumped up to it from a depth of a thousand meters or more through a “cold-water pipe” that extends those many meters from the surface of the ocean to its depths.
Although OTEC is a promising technology, its commercialization has been limited by a number of technical challenges. Among other challenges are the fabrication, transportation, and installation of the cold-water pipe, which can exceed 10 meters in diameter and 1000 meters in length.
Conventional large-diameter pipes of great length are usually fabricated by forming short cylindrical sections and then joining them end to end. Typically, the pipes are made from metal, concrete, or composite materials. Pipes that are made of metal are constructed of rolled sheets that are welded and internally stiffened via rings. This requires extensive skilled welding labor and results in a very heavy pipe. Concrete pipes are slip formed and pre-tensioned. Joining the circular segments is very difficult and the resulting pipe is, of course, exceedingly heavy.
Composite pipes constructed to date comprise a filament-wound inner laminate, a core sprayed over the inner laminate, and an outer laminate that is filament wound over the core. The length of the mandrel (and the ability to remove the composite from the mandrel) limits the length of a unitary section of composite pipe to only a few meters. Many sections must be joined, end-to-end, to create an cold-water pipe suitable for the OTEC application. The sections are butt joined with an overwrap. Each of the many required butt joints is a weak spot in the pipe. Furthermore, the joining process is very labor intensive.
Once a long-length, large-diameter pipe is manufactured, it must be transported to the OTEC plant. Whether transported in assembled form or in segments, transportation is expensive and raises safety concerns. Once on site, the pipe must be up-ended (or assembled and then up-ended) for submersion into the ocean. This is also a difficult and potentially quite risky procedure.